A response delay in an exhaust gas measuring device for an internal combustion engine consists mainly of: a first-order or high-order delay element caused by the structure and/or electronic circuits of the device; and a dead time element including a transfer time interval that is a time interval required for exhaust gas discharged from the internal combustion engine to reach the exhaust gas measuring device.
The response delay is preferably as small as possible, and therefore for example, a delay correction operation disclosed in Patent Literature is used to improve the first-order or high-order delay element.
On the other hand, the dead time element is improved by minimizing not only the length of an exhaust gas circulation path in the exhaust gas measuring device, but the length of a sampling pipe for sampling the exhaust gas of the internal combustion engine to introduce the sampled exhaust gas into the exhaust gas measuring device.
Nevertheless, for example, in the cases such as a comprehensive exhaust gas measuring system using multiple types of exhaust gas measuring devices, due to the difference among the lengths of sampling pipes to the respective exhaust gas measuring devices, in particular, response components affected by dead time elements are not uniform, and without taking measures, it may be difficult to compare respective measurement results.
For this reason, usually, it is adapted to flow reference gas from introduction ports of the sampling pipes, and measure time intervals from the time of the start of the flow to times when the respective exhaust gas measuring devices detect the reference gas (specifically, times when measurement results rise to 50%). In addition, it is also adapted to set the measured time intervals as response delay time intervals (dead time intervals) specific to the exhaust gas measuring device as well as on the basis of the response delay time intervals, correcting the measurement results of the exhaust gas measuring devices for synchronization, and contribute to the comparison of the measurement results and exhaust gas analysis, respectively and correspondingly.
Specifically, by on the basis of the slowest response exhaust gas measuring device, making a correction to delay the measurement result outputs of the other exhaust gas measuring devices, the measurement results of the respective exhaust gas measuring devices are synchronized.
However, as a result of making careful examination in order to further increase measurement accuracy, the measurement results of the respective exhaust gas measuring devices mutually deviated from one another in some cases. The present inventor has first found that the cause for the deviations lies in the fact that when setting the response delay time intervals, the reference gas was flowed from the introduction ports of the sampling pipes at a constant pressure such as 1 atm.
In other words, the present inventor has identified the cause as follows. That is, in actual exhaust gas measurement, pressures inside the sampling pipes vary in response to a variation in the discharge amount of the exhaust gas, or the like. As a result, in response to the pressure variations, the exhaust gas inside the sampling pipes compresses and expands, and the dead time elements and first-order delay elements vary depending on each of the exhaust gas measuring devices. As a result, deviations from the already fixed response delay time intervals occur.
The variation in response delay time interval may cause a problem even when the number of exhaust gas measuring devices is one. For example, in a split flow dilution system, part of exhaust gas is sampled to measure the concentration of a specific component, and when measuring the amount of the specific component, it is necessary to multiply the concentration measured by an exhaust gas measuring device by the flow rate of the exhaust gas discharged at the same time. This is because when doing this, in the conventional system in which a response delay time interval of the exhaust gas measuring device is fixed, a measured flow rate cannot be synchronized, and consequently an error occurs in the measured amount of the specific component.